In FM wireless communication it is known that the multipath transmission of a frequency-modulated signal radio wave reduces the quality of communication. This problem in FM communication is of significant inportance particularly in case the FM receiving unit is mounted in a vehicle such as a motorcar, since all the scattering objects on the earth may cause the multipath transmission of radio waves and a motorcar runs through or between such objects.
In a prior art FM receiver, means for avoiding or reducing the multipath noises is provided which comprises an array of receiving directional antennas having directions of maximum sensitivity different from each other, a control circuit adapted to detect multipath noises in a demodulated FM wave in order to generate a control signal when a multipath noise appears in the wave, and a switching means responsive to the control signal for selectively connecting suitable one of said antennas to a front end circuit of the FM tuner.
In an FM receiver of the above-mentioned type it is a problem that pulse noises of high frequency originated from for example electric discharges of ignition plugs of a motorcar may be caught by the antennas in addition to the multipath noises. Accordingly, pulse noises or high frequency noises as well as multipath noises are included in the demodulated signals in the FM tuner, with the result that said control circuit generates the control signal in response to not only multipath noises but also the noise pulse caused by the ignitions whereby the receiving antennas are needlessly switched one to the other whenever the noise pulses are caught by the antennas.
It is thus the object of the present invention to provide an FM receiver with receiving directional antennas in which the above-mentioned drawback is eliminated and the noise due to the multipath noises can be effectively suppressed whitout undue operation caused by pulse noises other than the multipath noises.